1. Field of the Invention
This invention relates to electronic musical tone synthesis and in particular is concerned with the selection of harmonics to imitate intramanual coupling.
2. Description of the Prior Art
Intramanual couplers are commonly used in the implementation of both theatre and concert type organs. Intramanual couplers are used to cause a selected combination of notes to sound for each actuated keyboard switch. Concert organs usually have intramanual couplers designed as octave couplers such as a 16-foot or a 4-foot coupler. If the 4-foot coupler is actuated, for example, then each note played on the keyboard will simultaneously cause the same tone to be played one octave higher than the actuated note. The use of intramanual couplers permit the musician to produce a large ensemble of notes while actually keying a relatively small number of keyswitches.
Intramanual couplers were skillfully employed in the design of theatre organs to produce a very large number of stops from a comparatively few ranks of pipes. In this design, which is called unification, the intramanual coupling is selectively implemented for an individual rank of pipes rather than for the entire keyboard as is the case for a concert organ. For example, the set of intramanual couplers for a tibia rank of pipes are frequently implemented to provide stops at 8', 4', 22/3', 1 3/5' and 1' pitch.
Intramanual couplers can, at least theoretically, be easily implemented in a digital musical tone generator or in an analog musical tone generator. An example of an electronic intramanual coupling arrangement is described in U.S. Pat. No. 3,697,661 entitled "Multiplexed Pitch Generator System For Use In A Keyboard Musical Instrument." In the disclosed system, the keyboard switches are scanned by means of a time division multiplexing arrangement. The intramanual coupling is implemented by delaying a pulse associated with an actuated keyswitch and reinserting the pulse at a later time slot corresponding to the desired intramanual coupling spacing.
A practical problem arises when one implements intramanual coupling in most types of digital tone generators. Each actuated intramanual coupler requires an additional set of tone generators that can be assigned to the keyboard having the intramanual couplers. The digital tone generators are a relatively expensive subsystem of a digital musical tone generator and it is costly to increase the number of these generators. Herein is an apparent paradox. Intramanual couplers were originally used to inexpensively expand the tonal resources of an organ. While analog organs can exploit this economical tone expansion scheme of intramanual couplers, the use of intramanual couplers for a digital musical tone generator may be a luxury subsystem.
A system design intended to imitate the tonal response of an intramanual coupler for a unified organ design has been employed in the implementation of digital musical tone generators. The underlying scheme is one that is called harmonic suppression. In this scheme a 4-foot stop is obtained by using a waveshape in which all the odd-numbered harmonic components are eliminated. A 22/3-foot stop is obtained by using a waveshape corresponding to the third harmonic sequence of the 3,6,9,12,15, . . . , harmonics. A 2-foot stop is obtained by using a waveshape corresponding to the fourth harmonic sequence of the 4,8,12,16, . . . , harmonics.
In U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer" a system is described whereby the tonal effect of unified stops is obtained by storing sets of harmonic coefficients having the appropriate missing harmonic coefficients.
In U.S. Pat. No. 4,286,491 entitled "Unified Tone Generation In A Polyphonic Tone Synthesizer" a system is described for creating the tonal effect of unified stops by the combination of three master data sets each of which corresponds to a period of the generated musical tone. The three master data sets are computed separately from stored sets of even and odd harmonic coefficient values. The master data set values are combined using their symmetric properties and are transferred sequentially to a digital-to-analog converter in repetitive cycles at a rate proportional to the unison pitch of the corresponding keyboard note to produce the tone color of a combination of unified tones.
Harmonic suppression does not provide the identical tonal effect of intramanual coupling for a mutation coupler such as a 22/3-foot or 1 3/5-foot coupler. Harmonic suppression provides an exact third or fifth harmonic base tone while intramanual mutation couplers are implemented to actuate the nearest musical note to the true third or fifth harmonic base note. Thus harmonic suppression schemes provide an approximation to the tonal effects produced by mutation intramanual couplers.